1. Technical Field
The invention relates to printing. More particularly, the invention relates to multilayer printing with white-balance.
2. Description of the Background Art
In photography and image processing, color balance is the global adjustment of the intensities of the colors, typically red, green, and blue primary colors. An important goal of this adjustment is to render specific colors, particularly neutral colors, correctly; hence, the general method is sometimes called gray balance, neutral balance, or white balance. Color balance changes the overall mixture of colors in an image and is used for color correction; generalized versions of color balance are used to get colors other than neutrals to also appear correct or pleasing.
Image data acquired by sensors, either film or electronic image sensors, must be transformed from the acquired values to new values that are appropriate for color reproduction or display. Several aspects of the acquisition and display process make such color correction essential, including the fact that the acquisition sensors do not match the sensors in the human eye, that the properties of the display medium must be accounted for, and that the ambient viewing conditions of the acquisition differ from the display viewing conditions.
The color balance operations in popular image editing applications usually operate directly on the red, green, and blue channel pixel values, without respect to any color sensing or reproduction model. In shooting film, color balance is typically achieved by using color correction filters over the lights or on the camera lens.
Sometimes the adjustment to keep neutrals neutral is called white balance, and the phrase color balance refers to the adjustment that in addition makes other colors in a displayed image appear to have the same general appearance as the colors in an original scene. It is particularly important that neutral, i.e. gray, achromatic, and white, colors in a scene appear neutral in the reproduction. Hence, the special case of balancing the neutral colors, sometimes gray balance, neutral balance, or white balance, is a particularly important, perhaps dominant, element of color balancing.
A particular problem occurs when an image is to be printed on a colored or off-white substrate. In such cases, the image can take on distinct color cast. In the art, the influence of an off-white or colored substrate is countered by first printing a layer of white ink on the substrate, thus establishing a neutral base upon which the image can be formed. For example, Vutek PressVu, QS, GS, and HS printers all have the ability to print multiple layers onto colored media, first printing one white layer for a consistent background and then printing other layers for the standard CMYK (cmykW) image. These printers rely on the white ink's pigment and opacity to deliver a solid and consistent white base on which a profiled colored image can be printed.
When printing on strongly colored media, the white ink alone may not have sufficient opacity without being printed in large, costly quantities. For example, when printing on a strong red sheet a single layer of white may appear to have a slightly pink hue. Adding a second layer of white may cure this problem, but at the cost of throughput and ink.
An alternative, but unsatisfactory, solution to this problem is time-consuming re-profiling for strongly colored media and avoiding solid white areas.
It would be advantageous to improve white-balance when printing on colored media, while minimizing the time and use of costly materials required by present approaches.